Radio transmitting system



April 3, 1934. v. E; TROUANT RADIO TRANSMITTING- SYSTEM Filed June 18, 1927 2 Sheets-Sheet l Allllllllfi- IIIIIIIIIFL Sa/uro/ec/ INVENTOR my/7E 7701/40]? ATTORNEY April 4- V. E. TRouANi' Y K 1,953,140 RADIO TRANSMITTING SYSTEM 2 Sheets-Sheet 2 Filed June 18, 1927 WITNESSE I |NVENTOR 5%,

1 /77/75. Fol/am.

.aflgmlsmn fi v ATTORNEY v Patented Apr. 3, 1934 RADIO TRANSMITTING SYSTEM Virgil E. Trouant, Wilkinsburg, Pa., assignorto Westinghouse Electric & Manufacturing Company, a corporation of Pennsylvania Application June 18, 1927, Serial No. 199,677 Claims. (01. 179-171) My invention relates to radio transmitting systems, and it has particular relation to systems of the type in which the radiated power is main tained'constant while the frequency thereof is 5 varied in order to transmit signals. a

One, object of my invention is to provide appropriate means and circuit connections whereby the carrier wave of a radio transmitting system may be modulated, as to frequency, in a smooth, continuous manner, eliminating the abrupt changes incident to frequencymodulation systems of the prior art. Anotherobject of my invention is to provide a modulating system particularly. adapted for v use in connection with radio transmitting installations of the type in which the frequency of 1th: carrier wave is controlled by a master-oscila or. I Another object of my invention is to provide a modulating system that is equally effective for both voice andtelegraphic-signal transmission. Another object of my invention is to provide 7 a modulating system in which the "frequency of a. piezo-electric crystal-controlled master-oscillator may be varied in accordance with audio or other signals without the Y expenditure of any considerable amount of power.

Still another object of my invention is to' provide a modulating system that is substantially without time-lag in its response to the audio or other signal impulses that are being impressed thereon.

In the majority ofradio transmitting systems designed for telephonic communication, the sig- 5 nal is impressed on a constant-frequency carrierwave as a modulation of the amplitude thereof. In certain systems intended for telegraphic communication, the carrier wave is interrupted at signal frequency and, in others, the characters or spaces are represented by a frequency-shift in the carrier wave. Y

Systems of the latter type, previous to the invention disclosed in the copending application of Frank Conrad, for Radio transmitting system,

Serial No. 176,060, filed March 17, 1927, and assigned to the Westinghouse Electric and Manufacturing Company, had not proved very successful for the transmission of voice, largely because no means was available for shifting the 0 frequency of the carrier wave at a sufliciently high speed to follow the audio-frequency varia- 1011s. i

In the Conrad application referred to, disclosure is made that audio-frequency modulation of a carrier wave may be accomplished by directly altering the frequency that a piezo-electric crystal-controlled master-oscillator is generating and supplying through appropriate amplifiers, v to a radiating structure. In that application it is shown that, if vibrations at audio-frequency are supplied to the condenser-electrodes with which the piezo-electri crystal is associated in the input circuit of the master-oscillator, the ultimate carrier wave will be shifted in frequency corresponding to such vibrations, and that, by the use of receiving circuits having a very low decrement, it is possible to successfully receive the signals at distant points.

My present invention. is in the nature of an improvement over the invention disclosed in-the Conrad application, being directed j toward an electrical, rather than mechanical; control of the frequency generated by a master-oscillator.

In one aspect, my invention provides means for varying the response-frequency of a piezoelectric crystal utilized to determine the frequency generated by a master-oscillator. Specifically stated, the alteration of the responsefrequency is obtained by changing the characteristics of the network which associates the 30 crystal with a thermionic amplifier, either by altering the value of an impedance device connected in shunt to thecrystal, or by changing the potential impressed across the crystal, either through the amplifier tube or direct, by means of a grid-biasing battery.

The generated oscillations may be amplified and radiated. or may be utilized to control a harmonic generator which, in turn, may supply the radiation frequency. In the latter case, the frequency shift of the finally radiated carrier is a faithful copy of the percentage shift of the master-oscillator, and truly represents the modulation.

The novel features which I consider characistic of my invention are set forth with particu- 'larity in the appended claims. The invention, in its generic aspect, however, both as to its organization and its method of operation, to-

- gether with further objects and advantages thereof, will best be understood by reference to the following description, taken in connection with the accompanying drawings, in which Fig. 1 is a diagram of circuits and'apparatus illustrating one form of my invention particularly adapted to telegraphy,

Fig. 2 is a similar diagram illustrating a form particularly :adapted to telephony,

Fig. 3 is a similar diagram illustrating a modification of the form'shown in Fig. 2, in which the apparatus associated with the grid circuit oi the oscillator is simplified, I

Fig. 4 is a similar diagram illustrating a further modification in which the properties of ,an

induction coil with a saturated iron core are utilized,

Fig. 5 is a similar diagram of a further modification which is particularly adapted for use with crystals having two well-marked natural periods,

Fig. 6 is a similar diagram illustrating the application of the invention to combined telegraph and telephone uses, and

Fig. 'I is a similar diagram illustrating the manner in which frequency-control may be obtained by alternating the plate potential supplied to a crystal-controlled master-oscillator.

In the form illustrated in Fig. 1, the vacuum tube 1 is provided with a parallel-resonant circuit comprising a condenser 2 and an inductor 3 in the plate circuit thereof. The plate circuit is supplied with energy by the'usual B battery 4.

The grid circuit of the tube 1 includes an inductor 5 and a piezo-electric crystal 6, constituting a capacity in parallel with the inductor 5. A condenser 7 between the inductor 5 and the piezo-electric crystal 6 is of sufficient capacity to constitute a practical short circuit for currents of the frequency generated by the system. The condenser '7 acts as a stopping con- 'denser, preventing the potential of the battery 3 from being applied to the grid except through the grid resistor 9.

A portion of the inductor 5 is shunted by a circuit including the contacts of a relay 11, which is controlled in any desired} manner. indication of this control, a telegraph key 12 is illustrated.

In the modification illustrated in Fig. 2, a different form of shunt about a portion of the inductor 5 is shown. In this form, the impedance of that portion of the grid circuit, including the lower part of the inductor 5 is the result of several impedances inparallel, i. e. (1) the lower part of inductor 5, (2) the inductor 14 and (3) the impedance of the vacuum tube 15. The tube 15 is energized by the B battery 16 and the inductor 14 is a part of the plate circuit of the tube 15. The condenser 17 is like the condenser '7 in that it constitutes'a practical short circuit for the high frequencies present in the system. This condenser prevents the battery 16 from being short circuited by the inductor 14 and the shunted portion of the inductor 5.

The impedance of the tube 15 is controlled by a microphone 18, acting in a well known way through the transformer 19. A bias may begiven to the grid of the tube 15 by a battery 20 and, by this means, the average impedance of the tube 15 may be given any desired value.

In the form illustrated in Fig. 3 the vacuum tube 15 is not supplied with a separate inductor corresponding to the inductors'14 of Fig. 2, but the lower portion of the inductor 5 is made to serve as,the plate inductance for the tube 15. A condenser 21, similar in certain respects to the condenser 1'7, connects the lower portionof the inductor 5 to the filament of the tube 1. The battery 22, similar in some respects to the battery 16, is connected to the filament of the tube 15 and to a point betweenthe inductor 5 and the condenser 21. i

In the form illustrated in Fig. 4, the battery 24, which serves as a B battery-for the tube 15,

- is connected through a plate inductance 25 As an" choke-coil 5.

which serves not only for the plate inductance of the tube 15 butalso as the primary of a transformer, the secondary 26 of which is analogous to the inductor 5 in the figures already described. The core of this transformer is preferably maintained in a more or less saturated condition by the plate current of the tube 15.

In the form illustrated in Fig. 5, the inductor 5 is placed between the condenser 7 andthe grid of the tube 1. The impedance at the opposite side of the condenser 7 .is the impedance of two paths in parallel, i. e. the impedance of the plate inductor 14 and the impedance of the tube 15. The impedance is varied by the relay 18 acting to vary the bias supplied .by the battery 26 to the grid of the tube 15.

In Fig. 6, an adaptation of that form of the device shown in Fig. 2 to combined telephone and telegraph uses is illustrated. A similar adaptation of any. of the other forms of the invention to telegraphy, in addition to telephony, may be made. The impedance in shunt to the lower part of the inductor 5 includes two paths in parallel, i. e. that through the inductor 14 and that through the tube 15. Variations of the impedance of the tube 15 are produced by the microphone acting through the transformer 19. The battery 20 in series with the secondary of the transformer 19, instead of being directly connected to the filament, as in, Figs. 2 to 5, is connected thereto through a high resistance 29.

In Fig. '7, the oscillating tube 1 includes, in its grid circuit, only the crystal 6 and the grid resistance 9, in series with the C battery 8. A separate battery for the tubes 1 and 15 is not used in Fig. "I, but the B battery 30 is caused to supply the two tubes in parallel. The usual choke coil 31 is in series with the battery 30 and has used for varying the amplitude of the oscillating currents. The tube 1 being the oscillation generator, the tube 15 is connected thereto as the usual modulator would be connected except that the common connection to the battery includes the resistor 32 as well as the choke coil 31.

The impedance of the modulator tube 15 is varied by the microphone 18 and transformer 19 changing the grid potential, as in the modifications already explained.

The battery 34 is analogous to the battery 20 and maintains a steady bias upon the grid of the tube 15. This bias may be abruptly changed by the relay 35 which shunts a portion of the battery 34 through the resistor 36 upon the closing of the telegraph key 3'7.

In the operation of that form of my invention shown in Fig. 1, the vacuum tube 1 generates oscillations the frequency of which is controlled by the response-frequency of the piezo-electric crystal and the tuning of the plate circuit 2-3. The response frequency of the crystal is, in turn, dependent, to a certain extent, upon the grid-potential of the tube 1, and upon the magnitude of the impedance afforded by the condenser '7 and the The response-frequency of the crystal can differ by only a very slight amount from the fundamental, or natural, frequency, thereof, the latter frequency being a function of crystal-dimensions.

It is, nevertheless, possible, by changing the characteristics of the network associating the crystal with thethermionic amplifier, to alter the response frequency in some instances by as much When the relay contacts 11 close, the impedance in the grid circuit is changed because a part of the inductance is shunted.- A corresponding change will, therefore, occur in the potential of the grid and will result in a change in the frequency, at which the system, as a. whole, oscillates. When the relay contacts 11' open, the impedance changes in the opposite direction, and the frequency, therefore, also changes in said opposite direction.

These changes in frequency are remarkably abrupt and are not accompanied ,by any perceptible change in the amplitude of the plate current of the tube 1. If, therefore, the system illustrated in Fig. 1 be associated with a sending antenna, either direct or through appropriate amplifiers, the radiations sent out will change abruptly in frequency upon the manipulation of the telegraph'key 12, but will not change to any noticeable extent in amplitude.

In Fig. 2, a similar action takes place. The operation of the microphone 18 causes a change in the impedance of the tube 15 and, therefore, a change in the combined impedance of the three paths of which this tube is one. The combined impedance of these three paths is a portion of the impedance in the grid circuit and its change will result in changes inthe frequency at which the system, as a whole, oscillates, as described in connection with Fig. 1. The changes in im-' pedance produced by the microphone are not abrupt, but are periodic, corresponding to voice frequencies. 1 The output from the system shown in Fig'. 2 will, therefore, be a current which is of very nearly constant amplitude but having a frequency which varies in accordance with'the voice.

A similar explanation applies to the system shown in Fig. 3. The impedance of the two paths in parallel, i. e. the tube 15 and the lower part of the inductor 5 changes with the action of the microphone and causes corresponding changes in the frequency of the output.

In Fig. 4, the inductance of the secondary 26 changes with the degree of saturation of the core. The magnetization of the core is maintained by the battery 24 and the winding 25 at such a point on the magnetization curve that the changes in impedance of the tube 15 resulting from the operation of the microphone 18 cause corresponding changes in the inductance of the winding 26. Changes in the inductance produce changes in the impedance of the grid circuit'which, as above explained, produce changes in the frequency of q the generated oscillations.

. Fig. 5 is similar to that already explained in connection with Figs. 2 and 3.. It is like Fig. 3 in that it uses only a small amount of inductance in the grid circuit of the tube'l and like Fig. 2 in that the inductor? is separated from the inductor 14 by a stopping condenser.

In the form illustrated in Fig. 6, the microphone produces changes in the frequency of the impedance coupled to said means, means mqenergy delivered by the plate circuit of the tube 1, in the manner already explained.

'I'he tele The system shown in Fig. 7 isbelieved to depend, for its operation, on the assumption that the modulation or the keying produces changes in graph key, by controlling the relay, causes abrupt changes in the potential of the grid of the tube 15 and, therefore, abrupt changes the plate voltage of the crystal-controlled oscil-- .lator tube 1, and, in consequence thereof, causes a varying amount of energy to be fed back through the tube to the crystal. Changes in the voltage impressed on the crystal are thought to produce changes in the response frequency thereof in the same manner as has been previously described in connection with the other figures of the drawings.

The changes in impedance of the tube 15 are produced by changing the grid potential, either through the action of the microphone 18 or through the action of the telegraph key 37. v

In the event that it is desired to transmit telegraphic and telephonic signals simultaneously, frequency-shift modulation may be utilized for one type of signals and amplitude-change for the other. In other words, telegraphy may be ac:- complished by shifting the frequency in the manner described in connection with Fig. 1, while voice may be impressed by the well known Heising modulation scheme upon a later amplifier stage, or vice-versa.

I have also found it feasible to control the frequency of a master-oscillator of the Hartley type, exemplified by Patent 1,356,763, by a systemof connections analogous in some respects to those previouslydescribed; In this modification the grid-filament portion of the main inductor may have connected in shunt thereto a circuit network equivalent to that at the 1m of the line ployed will be'apparent to those skilled in the art.

Myinvention is, therefore, not to be-limitedexcept as necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1., In an oscillation generator comprising an electronic device having an input circuit, means having a natural period of mechanical vibration connected in said input circuit whereby the average frequency of the generated oscillations is controlled thereby, means for frequency modulating the oscillations of said generator comprising an impedance coupled to said frequency control means and means including an electronic device for altering the electrical magnitude of said impedance as viewed from said generator in response to signal changes;

2. In an oscillation generator comprising an electronic device having an'input circuit," means havinga natural period of mechanical vibration coupled to said input circuit whereby the average frequency of the generated oscillations is con-' trolled thereby, means for frequency modulating the oscillations of said generator comprising an ing a thermionic device having a grid for altering the electrical magnitude of said impedance as viewed from said generator and means for varying the excitation applied to said grid in response to signals.

3. In an oscillation generator comprising an electronic device having an input and an output circuit, means having a natural period of vibration coupled to one of said circuits, an oscillatory circuit included in the other of said circuits and means electrically influencing the natural period of vibration of said first mentioned means in response to signals whereby to frequency modulate the current in said oscillatory circuit.

4. In an oscillation generator comprising an electron discharge device having an input and an output circuit, means having a natural period of mechanical vibration coupled to said input circuit to fix the average frequency of the oscillations generated, means for frequency modulating the oscillations comprising an impedance in circuit with said means and means for varying said impedance as viewed from said oscillator, in response to impressed signals.

5. In an oscillation generator comprising an electron discharge device having an inductive input circuit and an output circuit, means having vmcan E. ,TROUANT. 

